refactor: improve code and test coverage for MapReduce example

[alxkm]

• Renamed mapreduce method to countWordFrequencies for improved clarity and accuracy
• Improved internal word splitting by using trim() and split("\\s+") for better whitespace handling
• Replaced manual string building with Collectors.joining(",") for cleaner reduce phase logic
• Added parameterized tests

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.56%. Comparing base (350f149) to head (7f5900d).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6348      +/-   ##
============================================
- Coverage     74.56%   74.56%   -0.01%     
  Complexity     5451     5451              
============================================
  Files           682      682              
  Lines         19036    19032       -4     
  Branches       3690     3689       -1     
============================================
- Hits          14194    14191       -3     
  Misses         4277     4277              
+ Partials        565      564       -1     

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